It has become increasingly important to maintain communications connectivity in the presence of transmission system failures. To this end, ring interworking arrangements have been proposed for transporting communications circuits between bidirectional line-switched rings. Ring interworking is essentially a dual feed of communications circuits from a first ring to a second ring. The dual feeding takes place across two different sites, each with ring nodes for both the first and second rings, in order to provide the physical diversity necessary to enable the cross-ring communications circuits to survive a failure of one of the two shared sites. The second ring performs a receive switch based on some parameter or set of parameters of the two signals being fed from the first ring. For the other direction of the same end-to-end cross-ring communications circuits, the second ring dual feeds communications circuits to the first ring. The first ring performs a receive switch of the two signals being fed from the second ring.
A problem arises when there is a need to groom the inter-ring communications traffic by way of intervening wide-band digital cross-connect systems (DCS), multiplexers (MUX) or the like, where the grooming is being done at a different digital signal level, i.e., bit rate, than is being used in the rings. Examples are DS1 digital signals in DS3 signals and VT SONET signals in STS-1 SONET signals. In such arrangements, if a failure occurs incoming to the DCS, MUX or other grooming apparatus from one ring, it will not be recognized by the other ring because the inter-ring grooming apparatus will only insert the DS1 or VT failure indication and not the DS3 or STS-1 failure indication. Consequently, the inter-ring groomed signals, i.e., DS3s or STS-1s, passed off to either ring could appear to be "good" but could, in fact, contain corrupted or failed lower level digital signals, i.e., DS1s or VTs.
One attempt at avoiding the problems associated with inter-ring grooming of lower level digital signals in the higher level digital signals employed in interworking path-switched rings is described in a contribution to T1 Standards Project T1X1.2 entitled "SWB Ring Interconnection Architecture Issues and Proposed Interim Solutions", T1X1.2/93-013, dated Mar. 1, 1993. If the path-switched ring solution proposed in the noted Contribution T1X1.2/93-013 were to be applied to interworking bidirectional line-switched rings, the result would be an inefficient arrangement requiring the use of additional service bandwidth between the shared interworking nodes, the use of additional equipment in the nodes and the use of more interface and grooming capacity in the inter-ring grooming apparatus.